Phosphodiesterase 4 inhibitor (hereinafter abbreviated as PDE4 inhibitor) is a drug that suppresses the action of the enzyme phosphodiesterase, which degrades cyclic AMP (hereinafter abbreviated as cAMP) and, as a result, has effects of increasing intracellular cAMP concentrations to relax smooth muscle and suppressing activation of inflammatory cells. Therefore, the PDE4 inhibitor is used as a therapeutic agent for bronchial asthma, chronic obstructive pulmonary disease, allergic dermatitis such as atopic dermatitis and contact dermatitis, and the like. Example reports on PDE4 inhibitors are described below. Patent Document 1 discloses that 1,8-naphthyridine derivatives are effective for asthma. Patent Document 2 discloses an eye ointment containing roflumilast. Patent Document 3 discloses a method for treating inflammatory skin disease and allergic skin disease by locally administering a hydroxyindole compound. Patent Document 4 discloses a therapeutic agent for pruritus that contains a piperidine derivative.
A PDE4 inhibitor used for the treatment of an allergic dermatitis is often used as a topical product, which can be directly acted on the skin, in particular, an ointment. In general, an ointment is produced by adding a drug substance and ingredients such as an emulsifier, a solvent, a preservative, a moisturizing agent, and an absorption enhancer to an oily base or a water-soluble base, and mixing these ingredients uniformly. However, some drugs may be hardly absorbed into the skin only by allowing these drugs to exist uniformly in the ointment. In such cases, a method of adding a absorption enhancer to the ointment or suspending or dissolving a drug in a water-soluble or lipid-soluble medium and then kneading the suspension or solution into a base with other additives. For example, Patent Document 5 discloses an ointment produced by dissolving tacrolimus, which is an immunosuppressive agent, in propylene glycol by heating and mixing the solution with paraffin and petrolatum acting as bases, and isopropyl myristate acting as an absorption enhancer. Patent Document 6 discloses an ointment produced by dissolving flurbiprofen or indomethacin, non-steroidal anti-inflammatory agents, in 2-(2-methoxy-1-methylethyl)-5-methylcyclohexanol and mixing the solution with a petrolatum base. Patent Document 7 discloses a water-in-oil ointment produced by dissolving a drug such as azelastine hydrochloride, which is an anti-allergy drug, in propylene glycol and mixing the solution with a white petrolatum base and isopropyl myristate or the like acting as an absorption enhancer.    [Patent Document 1] Japanese Patent Laid-Open No. 2001-192385    [Patent Document 2] National Publication of International Patent Application No. 2005-529930    [Patent Document 3] National Publication of International Patent Application No. 2005-537262    [Patent Document 4] Japanese Patent Laid-Open No. 2005-47909    [Patent Document 5] Japanese Patent Laid-Open No. 5-17481    [Patent Document 6] Japanese Patent Laid-Open No. 8-165251    [Patent Document 7] Japanese Patent Laid-Open No. 2005-29541    [Patent Document 8] WO99/37622